It is well known in the field of oil well analysis, that reliable determination of the macroscopic neutron absorption cross-section (frequently referred to as Sigma) of the materials surrounding a borehole penetrating a geological formation can be of great assistance in well evaluation. For example, since the formation fluids are normally either salt water (having a relatively high Sigma) or hydrocarbons (having a smaller SIGMA), knowledge of Sigma enables the recognition of oil versus water in the formation. The same differences in Sigma may be used to help in the identification of the oil/water interface in the formation. Additionally, if two logs of Sigma are taken at different times, the production of the hydrocarbon bearing formation may be monitored. Permeable zones of a hydrocarbon bearing formation may be distinguished from less permeable zones by examination of the log of Sigma since the more permeable zones will be invaded by the mud filtrate (usually saline water having a Sigma much greater than that of the displaced hydrocarbon). And finally, Sigma is frequently used as a shale indicator. As a result of these many uses of Sigma, a variety of techniques have been developed to enable determination of this important formation characteristic.
For example, U.S. Pat. Nos. 3,566,116; 3,691,378; and 4,055,763 illustrate variations of one such technique in which a pulsed neutron source is utilized to repetitively irradiate the formation with a burst of fast neutrons in order to permit a time evaluation of the neutron population in the resultant neutron cloud. Typically, this evaluation is accomplished by detecting capture gamma rays which result when thermalized neutrons of the cloud are captured or absorbed by a nucleus of a constituent element in the formation. In such a time evaluation, advantage is taken of the fact that the neutron cloud density decays exponentially, with the characteristic decay time being a function of the macroscopic neutron absorption cross-section of the formation. The macroscopic neutron absorption cross-section is the sum of the neutron absorption of the elemental constituents of the formation and of its contained fluids.
One limitation that the pulsed neutron techniques for determining Sigma have encountered is their inability to properly determine Sigma in a formation containing large amounts of naturally radioactive elements such as thorium, uranium and potassium. Accumulations of one or more of these radioactive elements may produce a gamma ray background that obscures the desired information relative to the neutron cloud established by the pulsed neutron source. Unfortunately, accumulations of naturally occuring radioactive elements are often encountered in a producing well. It has been found that radioactive particulates which may be found in formation fluids tend to be filtered out and accumulate at the well casing perforations through which the formation fluids are flowed to create a naturally radioactive accumulation that decays to produce a relatively high gamma ray background which interferes with the detection method of the pulsed neutron technique. Thus, information regarding Sigma and oil/water movements in the very formation zones of greatest interest may be unavailable due to this obscuring background.
An additional limitation with the pulsed neutron technique is encountered in wells that have fresh water in the well borehole. In such a circumstance, some neutrons from the pulse are thermalized and linger in the fresh water of the borehole, giving rise to an interfering "diffusion" background. This effect of course does not occur in those boreholes having saline water since the chlorine is a strong neutron absorber and rapidly scavenges the diffusion neutrons. The "diffusion" background is a particularly bothersome phenomenon for the pulsed neutron technique since the determination of the characteristic decay time following the neutron burst relies on the detection of neutron fluxes whose intensities decrease with time to relatively small values. As a result, the "diffusion" background becomes large relative to the neutron flux of interest so as to obscure the information bearing signal.
In addition to those techniques which make a time analysis following a neutron burst, at least one other technique for the determination of the macroscopic neutron absorption cross-section has been attempted. This technique utilizes a continuous neutron source to irradiate the formation with a relatively high flux of relatively high energy neutrons. The spatial distribution of the resultant neutron cloud is concurrently examined by a pair of radiation detectors spaced from the source. One of the detectors responds to the presence of thermal neutrons while the other is responsive to the presence of epithermal neutrons. Advantage is then made of the fact that the spatial distribution of the epithermal neutrons is functionally related to the porosity of the formation while the spatial distribution of the thermal neutrons is functionally related both to the porosity of the formation and to the macroscopic neutron absorption cross-section of the formation.
A first example of a technique which has attempted to take advantage of this fact is the technique disclosed in U.S. Pat. No. 2,667,583. There it was proposed to detect the interface between a brine native to earth formations and liquid hydrocarbon by subjecting the earth formation to a fast neutron flux, and to produce two electrical signals dependent repectively upon the detection of neutrons having thermal and greater energies (thermal neutrons and epithermal neutrons), and the detection of neutrons having greater than thermal energies (eptithermal neutrons). It was suggested in that patent that the resultant signals be subtracted one from another with their difference being indicative of a qualitative indication of the presence of salt water or hydrocarbon containing formations.
U.S. Pat. No. 2,971,094 suggests a closely related although improved technique in which a first detector is sensitive substantially solely to neutrons having epithermal energies, while a second detector is sensitive substantially solely to thermal neutrons. In the practice of the disclosed invention there, the signals from the detectors may be either subtracted to form a difference or divided to form a ratio. Further, it is suggested in that patent that a strong thermal neutron absorber be artificially introduced into the earth formations by incorporating a strong thermal neutron absorber in the drilling fluid in order to produce an overall increase in the thermal neutron absorbing characteristics of the earth formation.
As a further example of a prior art technique which utilizes a continuous source of neutrons to bombard the formation, U.S. Pat. No. 3,435,217 proposes that a first detector record thermal neutrons, that a second detector record epithermal neutrons and that a third detector record capture gamma rays. In that patent, it is taught that the epithermal neutron signal is dependent upon the porosity but independent of the chemistry of the formation, while both the thermal neutron and the capture gamma ray signals are dependent upon the chemistry of the formation as well as the porosity of the formation. These signal dependencies are used to advantage to remove the porosity dependence of the thermal neutron and capture gamma ray signals by their appropriate combination with the epithermal neutron signal. In making this combination, a graphical technique is utilized for converting the epithermal signal into a porosity component of the thermal neutron signal and a porosity component of the gamma ray signal. These porosity components are then removed from the thermal neutron signal and from the gamma ray signal, respectively, by taking the appropriate ratios. Finally, the resultant chemistry dependent but porosity independent thermal neutron and gamma ray signals are referenced to shale.
Finally, U.S. Pat. No. 4,005,290 discloses a technique in which the ratio formed from a pair of epithermal neutron detectors is compared with the ratio of the signals derived from a pair of thermal neutron detectors in order to determine a differential therebetween, said differential yielding a qualitative indication of the salinity of the formation.
While each of the above techniques is free of the mentioned limitation of the previously described timing technique, since they detect neutrons as opposed to gamma rays and are therefore not influenced by gamma ray backgrounds, and since the neutron flux of interest is large compared to the "diffusion" background so that the "diffusion" neutron flux from fresh water filled boreholes is not a serious difficulty, they have not been well received in the commercial world primarily due to their unfortunate inability to provide satisfactory quantitative values of Sigma as opposed to general qualitative values. This inability stems from the lack of a complete understanding of the physics inherent in the spatial distribution of thermal and epithermal neutrons relative to the location of the irradiating neutron space. It is evident that recognition of the proper functional relationship between the spatial distribution of the thermal and epithermal neutrons and the Sigma of the formation is necessary before such techniques can be made to yield quantitative as opposed to qualitative determinations of Sigma.